The Knife, also known as the Butterfly Knife or Balisong, is the default melee weapon for the Spy. It is a foldable butterfly knife with a handle clip and clip-point blade.

When holding the weapon while within melee-range at a 180° radius behind an enemy, an animation will play wherein the Spy holds the Knife aloft in a striking position. Attacking at that point will cause the player to backstab the enemy, killing them in one hit, unless the player backstabbed is a Spy who's currently holding the Dead Ringer out, a Sniper equipped with the Razorback or a player in any kind of invulnerability, such as an Übercharge. In any other instance, such as when used from the side or front, the Knife will deal normal damage, which is comparatively reduced when compared against other stock melee weapons. In addition, random critical hits do not occur when using the Knife.

Bugs

When the Knife is thrust forward during an attack, a stray polygon can be seen coming out of the Spy's wrist.

Trivia

The original drawing animation for the Knife featured a much more elaborate spin. It was cut, however, for being too long and complicated. Valve feared that this would draw the player's eye away from their target during a fight. This animation can still be seen in the teaser trailer for Team Fortress 2 (from 1:33 to 1:35).

The model of the Spy's butterfly knife is inaccurate to its real life counterpart, as it is missing tang pins, which keep the blade from moving around.

The Spy seems to open his knife with a shortened version of the Basic Vertical.